Yes, in some organization velocity is the popular method to approximate the cost. Your calculation is correct. Now for your next question is how velocity helps in costing:

Based on the historical analysis of velocity, how many story points a team can deliver is approximated. And the number of ideal hours also approximated for a Story points. Now you know the per hour cost and from this information, the cost is approximated.

Note: It is not a good idea to judge velocity from the first sprint, it just gives an approximate. You need to monitor team velocity sprint by sprint, and once you observe that it becomes almost constant after a few sprints, you may need to change your plans.